International Maritime Organization announced an MARPOL (Prevention of Pollution from Ship) rule in order to reduce atmospheric pollution from a vessel and makes an effect to reduce atmospheric pollution to a 5 to 20% level of current emission quantity. Accordingly, worldwide researches are being carried out in order to replace existing heavy fuel oil-series bunker oil, that is, fuel for vessels, with Liquefied Natural Gas (LNG).
In line with this regulation, a small vessel using LNG as fuel has emerged in the coast of Europe. A small vessel has an LNG fuel tank having a small capacity and uses fuel gas of low pressure because the small vessel uses a small amount of LNG and has a short shipping service distance. In contrast, a large vessel needs to use a 2-cycle engine using a direct driving method in order to increase fuel efficiency because the large vessel requires an LNG fuel tank having a large capacity, uses a very large amount of fuel, and has great propelling force.
In order to obtain natural gas of high pressure, a high-pressure heat exchanger for vaporizing extremely low temperature materials is necessary. Heat exchangers satisfying this requirement include a Shell & Tube type heat exchanger developed about 100 years ago and now being used, a coiled tube type heat exchanger having a plurality of tube bundles, and a fin-tube type heat exchanger in which a pin is attached to a tube.
Furthermore, as a contrary concept, in many plants, in order to cool or condense a process gas having a high temperature and high pressure, a process of making a cold source, such as water or a refrigerant, flow into the secondary-side passage of a heat exchanger is chiefly used.
It is reported that an existing heat exchanger, such as the Shell & Tube type heat exchanger or the coiled tube type heat exchanger, is not suitable for a high-pressure LNG vaporizer for a natural gas-propelled vessel due to the following problems.
First, in the Shell & Tube type heat exchanger designed and fabricated to be used under a high pressure and extremely low temperature environment, fatigue fracture is likely to occur in a joint due to a difference between the heat capacity of a thick tube sheet used to withstand high pressure and a relatively less thick tube.
Second, the Shell & Tube type heat exchanger, etc. inevitably depend on the length of a tube in order to increase a heating area and require a large size and heavy weight due to low compactness.
Third, the size of an insulating boxes for clogging the entrance and exist of heat is further increased due to a large size, and thus a cost and an installation space within a vessel are greatly increased.
The problems are likewise generated in uses for vaporizing high pressure and low temperature materials and condensing or cooling high pressure and high temperature gas.
In addition to the above-described problems, there are some problems difficult to be solved in applying a Printed Circuit Heat Exchanger (PCHE) 10 that is known to be suitable for a high pressure heat exchanger shown in FIGS. 1 and 2. Accordingly, a system adopting the PCHE 10 has not yet emerged, and this problem is also present in a process gas cooler for a chemical plant as well as a natural gas vaporizer for a vessel.
First, icing, that is, a phenomenon in which a surface of a passage 2 through which a heat source for heating temperature of LNG flowing into a passage 1 at an extremely low temperature passes is frozen is generated. In order to avoid this phenomenon, a large passage having an average hydraulic diameter of 2 mm or more, that is, the size of a passage that can be fabricated by an existing photoetching process, is necessary. The passage having the large average hydraulic diameter makes it difficult to adopt the existing PCHE 10 because it does not fall within a technical limit and an economic limit of a photoetching process.
Second, since a heat source (or cold source) fluid used to vaporize or cool gas passes through the small passage 2 at low speed for a long time, a heat transfer rate is significantly reduced because a clogging phenomenon in which the passage 2 is clogged by an alien substance and a fouling phenomenon in which the inside of the passage is covered with fur become significant. In particular, this fouling problem is a problem that causes the largest claims after the PCHE has been commercialized as a cooler for the process gas of a plant. Furthermore, the problem can likewise occur even in high pressure LNG vaporizers that have not yet been developed and applied so far.
The PCHE 10 must be cleaned by high pressure washing or combustion using high temperature gas due to the clogging phenomenon and the fouling phenomenon and then reassembled. To this end, a corresponding vessel or plant must be stopped, but it is difficult to stop the entire system due to the nature of the system.
Accordingly, there is a need for a solution to the problems.